This invention relates to an apparatus and record carrier for a magneto-optical disc memory, and more particularly to an apparatus and record carrier for a magneto-optical disc memory which can process (record, reproduce or erase) information on any number of tracks or sectors at a time.
Further, this invention relates to a master disc cutting apparatus for forming a guide groove and a header signal in advance such as to enable the processing of information on any number of tracks or sectors at a time.
An apparatus and record carrier for a magneto-optical disc memory is an erasable optical information memory apparatus which records and erases information by utilizing what is called light induced thermo-magnetic effect, namely by irradiating a beam converged by a laser onto a recording medium of perpendicular magnetic thin film, cancelling magnetization temporarily by the heat of the optical energy, and setting the magnetization of the magnetic thin film in a predetermined direction by applying magnetization from outside, and which reproduces information by utilizing magneto-optic effect (for example, the Kerr effect and the Faraday effect), by rotating the direction of polarization of incident beam in correspondence with the direction of magnetization of the perpendicular magnetic thin film. An example of this apparatus is disclosed in pp. 208 to 214 of "Amorphous thin film disc for magneto-optical memory" by Y Togami et al, SPIE Vol. 329, Optical Disk Technology (1982).
When information is written successively into a rotating record carrier (magneto-optical disc) which has a perpendicular magnetic thin film provided on a disc substrate, it is necessary to record it at a constant pitch of, for example, 1.6 .mu.m while keeping a regular interval between said information and the information already recorded. It is also necessary to make it possible for a user to freely record, reproduce and erase information at a desired position of a perpendicular magnetic thin film.
On the other hand, in a non-erasable optical disc, in which a laser beam is irradiated to the recording film to make a minute hole by heating and evaporating the irradiated minute portion, information is recorded and reproduced along a guide groove which has been formed in advance on the disc. One example thereof is disclosed in Japanese Laid-open Patent Application No. 91536/1983 (corresponding to U.S. Pat. No. 4,607,358). In this kind of optical disc, a header signal such as a track address or a sector address is recorded in the guide groove in advance, information can be recorded or reproduced at a desired position of any number of tracks or sectors by conducting random access to a specific track or a specific sector in the track. Data which a user wants to record is recorded by making a minute hole in a recording film in the groove by irradiating a high-power laser beam.
However, in the case of a magneto-optical disc, an apparatus and record carrier for magneto-optical disc memory which reproduces information by utilizing the light induced magneto-optic effect represented by the Kerr effect and the Faraday effect and which records and erases the information by the light induced thermo-magnetic effect, a method for controlling information of any number of sectors such as described before has not been established.
For example, a method by which a header signal is recorded in advance by the direction of magnetization may be considered. However, by this method, recording has to be conducted by irradiating a laser beam onto each disc one by one, which would be inconvenient for mass production. In addition, there is a risk of mistakenly erasing a header signal. Further, one and the same apparatus does not suffice for handling a conventional non-erasable optical disc, and an optical disc for exclusively reproduction such as a digital audio disc (DAD) or a video disk (VD) and a magneto-optical disc.
In Japanese Laid-open Patent Application No. 218033/1983 a magneto-optical disc is disclosed in which a guide groove is provided in order to conduct secure tracking by the three-spot method and which records magnetic data in the guide groove. However, nothing is disclosed with respect to a header signal, nor is information control of any number of tracks or sectors taken into consideration.